Have you considered TI's WEBENCH Designer for the analog category? This is a free on line circuit design tool which includes design synthesis, component selection and Spice simulation. It also has thermal simulation and the ability to download the design into popular CAD tools. You can get to it at http://www.ti.com/webench.

Thank you for the advice. True...if I have lots of time, usually which is not the case, then a thorough investigation about the models would be possible. In general I tend to simulate to check the concept would or would not work. After ordering the prototype board and before those arrive, the time could be utilized for detailed simulation to be prepared for the mods on prototype...to avoid surprises and frustration :)

There is a glaring omission here. You assume nobody wants to use free software to check their PCB? I check things such as trace impedance, trace differential impedance, trace fusing current, trace dc resistance, resistance increase with temperature, estimate power distribution impedance, via current carrying capacity, via impedance and other things. Analog engineers look at their PCB characteristics too. I use Saturn and Ultracad freeware for this but we have an in house tool for traces impedances.

Is there a freeware equivalent of Hyperlynx, or would that be asking too much?

You don't always need the model. You might only care about the component tolerance. It depends upon the level of accuracy you are seeking. You can also use a spreadsheet for some calculations. Sometimes that's easier than SPICE. If you have a lot of unknows in you system, then should you spend a lot of time on the models?

Without some good test equipment all the modelling software in the world is useless if you can't prove your designs.

For free software: An old copy of ansoft serenade RF designer whichwas bundled with a copy of an antenna text book from the ARRL, LTspice and the excellent free antenna modelling program 4NEC2.

Hardware: This along with a Bird RF power meter and slugs, na HP465 precision RF power meter/with head and suitable attenuators, an old analog TEX 465 scope, a HP8558 spectrum analyzer, a HP8640 RF generator with built in freq counter and high power option and an MFJ antenna analyzer along with the solder iron mentioned previously by another individual is all I need for anything from HIGH power RF amplifiers, large multi-element external antennas to small etched internal antenna designs.

I use LTSpice for analog simulation and Icarus Verilog coupled with GTKWave for digital simulation. GTKWave is a top quality waveform viewer, and can compete in useability and features with the best commercial tools.

Icarus Verilog is a very complete and stable free Verilog simulator. It's a bit slow for large projects that require long time simulations, but totally adequate for many projects.

Another free simulator that I've got a lot of mileage with is the Verilator (Perl program), with turns synthesizeable Verilog into C++. If you have the need for speed, this tool will outperform the even the most expensive commercial simulator. Using the C++ model of the digital circuit that is output by the Verilator, you can model the external circuitry in C or C++. You can even make a DLL out of the model + behavior circuitry and call it with a GUI, scripting language, or whatever.

"Verilator should run any system with GCC and Perl. It is developed on 64-bit SuSE 9.3 and other users report success on Redhat Linux, HPUX, Solaris, and Windows NT under Cygwin (C++ only, no SystemC), and Microsoft Visual C++" And, I believe, OSX.

Thanks for the insight! This is a very important point you have mentioned. Using the incorrect models and differences between grounding in the practical scenario vs in simulation causes a lots of differences between simulation vs practical results. Earlier I used to use a ORCAD student's version [it was not free but less costly]...it was good. Currently I use LTSpice a lot and TINA sometimes. I like LTSpice over TINA, but as you have mentioned, I tend not to believe just on the simulation results but also to make a quick bread board circuit and scope to check if it would work as I expect it to.

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